An acrylic pressure-sensitive adhesive has generally been prepared by solution-polymerizing monomers mainly comprising a (meth)acrylic acid alkyl ester. However, from the restrictions on safety of organic solvents and environmental health, a method of preparing the pressure-sensitive adhesive by emulsion-polymerizing or photo-polymerizing the above-described monomers is recently widely used.
Such a pressure-sensitive adhesive is usually that an acrylic adhesive polymer is crosslinked with a crosslinking agent to increase a cohesive force at high temperature, thereby imparting an excellent heat resistance. However, since many of the adhesive components are constituted of combustible materials, there has been a difficulty that the pressure-sensitive adhesive cannot be used in fields of application requiring a flame-retardance together with a heat resistance.
To overcome this difficulty, JP-A-1-223187 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), for example, discloses a pressure-sensitive flame-retardant adhesive obtained by adding a flame-retardant or a flame-retardant assistant, such as a brominated material, chlorinated paraffin, aluminum hydroxide, antimony trioxide, etc., to a adhesive base polymer such as an acrylic polymer, a synthetic rubber, a natural rubber, etc.
However, it is necessary to add a large amount of a flame-retardant in order to exhibit a sufficient flame retardant effect by such a conventional pressure-sensitive flame-retardant adhesive. In this case, there is a problem that the heat resistance of the pressure-sensitive adhesive is decreased. Further, if the degree of crosslinking of the base polymer, such as a rubber, is increased in order to improve the heat resistance, the adhesive property thereof is decreased, so that it is necessary to add a large amount of a tackifier. In this case, there is a problem that holding power of the adhesive at high temperature is decreased in addition to increase of the cost.
In view of these circumstances, an object of the present invention is to provide an acrylic pressure-sensitive flame-retardant adhesive which has excellent adhesive performances such as adhesive force or holding at high temperature, and also has a self fire-extinguishing property for instantaneously extinguishing fire, that is, a flame-retardance; adhesive sheets obtained by forming the pressure-sensitive flame-retardant adhesive in, for example, a sheet form or a tape form; and a photopolymerizable composition for obtaining the above-described pressure-sensitive flame-retardant adhesive.